Investigation of Environmental Colloids

What are Colloids?

In physics lesson and chemistry lessons at school one learns about the three states of matter - solid,
liquid and gaseous. The laws of phase transition are also taught - melting, sublimation, and
condensation First, pure substances are discussed, then solutions are gone into. Solutions are
homogeneous mixtures of chemical substances that are dispersed on a molecular scale. The
existence of an intermediate state of matter, laying between the macroscopic volume phase and
molecularly-dispersed systems, had remained unknown until about one hundred years ago. In
systems showing this state, a component is dispersed in another one. However, the degree of
dispersion is lower than in simple molecular solutions. Systems of this kind,
colloids possess special
properties. These special properties make investigating colloidal systems difficult. They have limited
the success of colloid investigations for long periods of time.

Although already the alchemists had experimented on colloidal suspensions and the first systematic
scientific investigations on colloids have been performed as early as in 1856 (Faraday), the English
physicist Hedges wrote as late as in 1913: 'For some the word colloid evokes the imagination of
things which are poorly defined with regard to their shape, their chemical composition and their
physical properties and which are unsteady concerning their chemical behavior, i.e., things which are
mysterious and not controllable'. The German physico-chemist Ostwald (1853-1932) regretted the
lack of knowledge in the field of colloid research and called the world of colloids 'the world of the
neglected dimensions'.

It was not until the second half of the 20th century that a certain helplessness concerning the colloidal
state was overcome. However, the experimental difficulties that hampered the elucidation of the
behavior of colloids in the past still cause a certain tendency to avoid, if possible, the formation of
colloids in chemical experiments or, if this is impossible, to neglect the occurrence of colloids.

The special properties of the colloids result primarily from their large specific surface area which is
due to the fine-grained dispersion of the colloidal particles in the dispersing medium. The size of
colloidal particles lies in the range of 1 nanometer (1 millionth of a millimeter) to 1 micron (1
thousandth of a millimeter). The diameters of atoms and molecules are usually in the range of several
tenth of nanometers. Thus the percentage of atoms/molecules located at a phase boundary is
significantly higher in colloidal systems than in compact macroscopic solids. It is this high fraction of
atoms/molecules sitting close to a phase boundary what causes the special properties of colloids. One
of these properties is, for instance, the instability of many colloids (their tendency to coagulate). This
instability makes the experimental investigation of colloidal systems difficult. It causes one of the
most significant problems of colloid experiments: the unintentional change of the colloids during
taking the samples and during the measurements.

What well-known examples are there?

Examples of colloidal systems 'solid in a liquid' are paints, oozes of clay, latex suspensions or blood.
Milk is a 'liquid-liquid' colloid. Examples of macromolecular colloids are jellies, solutions of
polysaccharides, and glues. The expression 'colloid' results from the Greek word for glue. It was
coined by the Scottish chemist Graham in 1861. Graham is regarded as the founder of colloid
chemistry.

What are environmental colloids?

One of the most striking properties of colloidal solutions is their ability to scatter the light (Tyndall
effect), that they often are turbid or even entirely opaque. The latter refers, for instance, to milk.
Visibly turbid colloids play also a part in environmental research. However, more typical are different
colloidal solutions in environmental research: solutions that are not looked their particle content with
the naked eye. Many 'classic' colloid chemists would perhaps hesitate to call, for example, a clear
groundwater a 'colloidal solution'. Nevertheless, instruments that are more sensitive than the human
eye proof that
any natural water contains particles of the colloidal size range (1 nm to 1 µm). Clear
groundwaters show typically colloid concentrations of 0.01 to 1 mg/L. Typical particle number
concentrations of such waters are
1012 to 1014 particles per liter. The colloid concentrations of mining
waters or turbid river waters can be significantly higher (however, we do not refer to the so-called
'suspended matter' which has particle sizes of over 1 µm as colloids). The very low colloid
concentrations of many natural waters add further difficulties to the above-mentioned difficulties of
colloid research in environmental studies.

There are primarily two ways on which the inorganic colloid particles get into natural waters: stone
fragments are rinsed off by the weathering of the rock that is washed by the water and particles can
be formed within the water from truly dissolved substances by precipitation (formation of secondary
minerals). One can differentiate between several groups of colloids according to the generation
mechanisms of the particles. The first group include above all silicate colloids (layer silicates, quartz
etc.). Oxides, hydroxides or carbonates of iron, aluminum, manganese, or calcium belong to the
second group, the group of secondary mineral colloids. A further group of environmental colloids are
organic colloids (fulvic acids, humic acids, humin, polysaccharides). Their mechanism of generation
is the degradation of the remains of plants and animals. Finally, biogenic particles (bacteria, viruses,
fungi) form a fourth class of colloids in natural waters.

Why do we investigate environmental colloids?

The environment is usually treated as a diphase system in analyses of contaminant transport by
ground waters, mining waters, or surface waters and in calculational models for the prediction of this
contaminant transport. The contaminants (radionuclides, toxic heavy metals, organic poisons) are
distributed among the mobile aqueous phase and the immobile solid phase according to this
approach. The migration of the contaminants with a strong tendency to adsorb onto the rock of the
solid phase is delayed in comparison to the water velocity. Substances with very strong tendencies to
attach to the walls, for instance, should virtually not move at all in an aquifer. However, there are
increasingly observations and experimental results which indicate that the fraction of the solid phase
existing in suspended form as colloidal particles is not neglectable under certain conditions. It is
obvious that the contaminants that tend to be adsorbed onto solids are also adsorbed onto these
particles.

The total mass of the colloid particles in natural waters is usually small. However, the particles
possess a large specific surface area. Therefore, they offer a lot of sorption sites to the sorbing
contaminants. The association of contaminants with this additional mobile phase can increase the
velocity of contaminant transport. Vice versa, filtration effects or coagulation and precipitation of
colloids can delay the contaminant transport. The assessment of contamination incidents, the
prediction of migration scenarios in the vicinity of contaminated areas and of potential hazards as well
as the development of effective strategies of remediation for contaminated areas - they all require a
profound understanding of the migration-enhancing and the migration-delaying influences of the
colloidal particles on the contaminants. Background for our research is the problem of contaminant
migration in the surroundings of mines as e.g. the abandoned uranium mines in Saxony or the
assessment of the long-term behavior of radioactive waste disposal sites.

The typical difficulties of colloid research play also a role in the case of environmental colloids. They
explain why the question of colloids is still often neglected in estimations of contaminant transport.
The colloid-facilitated contaminant transport is regarded as elusive, the results of colloid experiments
are often poorly reproducible, deriving quantitative relationships is often not successful. However, it is
increasingly realized now that simple ignorance is the most unfavorable way of dealing with the
colloid problem. The American geochemist B. D. Honeyman stated 1999 in
Nature that the colloid-
facilitated transport of contaminants has become a sort of Gordian knot for environmental scientists.
We share this point of view and we also share Honeyman's opinion that this Gordian knot has not yet
been cut.